This invention relates to an apparatus for analyzing the dielectric properties of a sample by the use of parallel plate electrodes or single surface interdigitated pectinate electrodes.
It is well known that by measuring the dielectric properties of a sample as a function of temperature, valuable information can be gained concerning the physical and chemical properties of the sample. For many years such measurements have been made by placing a sample between parallel plate electrodes, applying an electrical signal to one of the electrodes (i.e. the excitation electrode) and measuring the electrical signal from the other electrode (i.e. the response electrode). The following equation is used:
C=e.sub.o e'A/d
where
C=Capacitance PA1 e.sub.o =Permittivity of Free Space (a constant) PA1 e'=Permittivity of Sample (being measured) PA1 A=Area of Parallel Plate Response Electrode PA1 d=Distance Between the Excitation and Response Electrode Plates
By measuring capacitance, the permittivity of the sample (e') can easily be calculated if the area of the parallel plate electrode and the distance between the excitation and response electrodes are known. However, a common dilemma when making these measurements is obtaining an accurate measurement of distance between the plates. This is because most measurements are made as a function of temperature, and the sample changes in dimension as the experiment progresses. However, despite this fact, prior parallel plate dielectric analyzers have usually assumed the distance between the electrodes to be the thickness of the sample at room temperature. Thus, as the material expands or contracts as a function of temperature, the measured values are in error by the factor: ##EQU1## In some instances this error is compensated by allowing for the coefficient of thermal expansion (CTE) of the material (assuming it is known with some accuracy). But this is not an accurate correction since the CTE changes as a material goes through its glass transition. The CTE also assumes zero force on the sample which is not practical when making dielectric measurements on solid samples.
All known instruments either apply a constant force to a sample initially and run the experiment in that mode (constant force), or set a plate spacing and let it remain constant during an experiment (constant distance). In the constant force mode, at elevated temperatures, when the sample melts, the two plates come together, short circuit, and the experiment is prematurely terminated. In the constant distance mode, if the sample melts, contact with the top plate is lost, and once again the experiment is prematurely terminated.
Another significant practical problem with conventional parallel plate dielectric analyzers arises because current analyzers use either steel or gold plated metallic plates. After a sample has passed its glass transition T.sub.G (point of interest), it begins to flow, and as it cools it can adhere to the highly-polished, precision-machined plates. Many times plates must be removed from the instrument to scrape samples off. The plates must then be reground to ensure parallelism for the next experiment. This can be a costly and time-consuming operation. One popular alternative is to use a thin release film (i.e. Teflon.RTM., a fluorocarbon polymer) to make sample removal easier. This film, however, influences the measurement of the dielectric properties and limits the experimental temperature to a temperature less than the melting point of the Teflon.RTM. release film. (Ceramic sensors with a gold conductor are used in single plate dielectric analyzers. See; Micromet product literature in the Information Disclosure Statement--Option S-60 dual function ceramic sensor for use in Micromet Eumetric System II microdielectrometer).
Accurate measurements of sample temperatures are also important since dielectric measurements are normally monitored as a function of temperature. In parallel plate dielectric analysis, typically a thermocouple is placed as close to the edge of the sample and plate as possible without contact, and the sample temperature is assumed to be that of the thermocouple (melting a sample on the thermocouple would require extensive clean up or disposal of the thermocouple after the experiment). Obviously, this temperature measurement is not as accurate as measuring the temperature of the sample directly. (In single plate dielectric analyzers it is known to incorporate a thermal diode in the electrode. See; Micromet product literature in the Information Disclosure Statement--Option S-1 integrated circuit dielectric sensor for use in the Micromet Eumetric System II microdielectrometer).
A parallel plate dielectric analyzer is needed which can vary the spacing between the electrodes as the sample expands, contracts, or melts in order to keep the electrodes in constant contact with the sample. As the electrode spacing is varied, the analyzer must also be able to sense the distance between them so that the dielectric calculations are accurate regardless of electrode spacing. A dielectric analyzer is needed which has electrodes which are easily replaced if their surfaces become marred. A dielectric analyzer is also needed which will give accurate temperature measurements of the sample.
Dielectric analysis using parallel plate electrodes is a powerful technique, however, it is primarily used to characterize the bulk properties of a material, in that the excitation signal is monitored through the entire thickness of the material. This constraint results in some critical limitations. Often times thick samples are of interest to be analyzed. In the parallel plate technique, the signal to noise ratio decreases as a function of increasing distance between the electrode plates. Larger plates could be uitlized to increase the area thereby increasing the signal however there does exist a practical limitation. Many times the surface of a material is to be analyzed. In polymer molding, skin effects are of interest due to faster cooling of the material's surface than its interior. The chemical and mechanical properties of the surface of the material are more indicative of its end use properties than the bulk properties. Coatings on a material surface are also of interest in dielectric analysis. Paints, adhesives, and copolymers often require analysis. A parallel plate measurement would detect the properties of the coating and its associated substrate in a bulk fashion. It is impossible to analyze surface characteristics by parallel plate analysis.
An alternate technique was developed and is commonly known which addresses the limitations of the parallel plate measurement. An interdigitated combed electrode is commonly used for obtaining dielectric measurements on surfaces of materials and fluids. Probes of this type have been used for many years as moisture detection devices. U.S. Pat. No. 3,696,360 to Gajewski, discloses an interdigited electrode for moisture sensing. In the past few years these interdigitated probe structures were adapted to measure dielectric properties of materials. See, Society for the Advancement of Material and Process Engineering Journal, Volume 19, No. 4, July/August, 1983. U.S. Pat. Nos. 4,710,550 and 4,723,908 both to Kranbuehl also disclose the use of single surface interdigitated pectinate electrodes for measurement of dielectric properties of materials. Another form of a single surface interdigitated dielectric sensor is disclosed in copending U.S. patent application Ser. No. 07/274,461 assigned to the assignee of the present invention.
In the single surface analysis technique a sample is placed on the electrode surface, an alternating electric voltage is applied to one "finger" or comb of the interdigitated fingers or combs of the electrode array, thereby inducing a current which passes through the sample and is measured at the other finger of the array. These two fingers are termed excitation and response electrodes respectively. In this fashion, the field only penetrates the surface of the material. The penetration depth of the alternating fields is approximately equal to the distance separating the fingers in the interdigitated electrode array. This technique is ideal for monitoring the dielectric characteristics of surfaces of materials as well as fluids, curing systems, adhesives, and relatively low viscosity materials.
Many limitations also arise when performing single surface dielectric analysis of materials. Most experiments require samples to be urged into contact with the electrode array by applying a constant force. As previously mentioned, the applied alternating field penetrates into the sample a finite distance. If the means to apply force to the sample penetrates this field area, accurate measurements are compromised. As with parallel plate analysis, samples are typically tested as a function of temperature. An analagous problem arises using single surface electrodes as with parallel plate electrodes. In a constant force experiment, at elevated temperatures, the sample begins to flow and the force application means drives toward the electrode array, penetrating the field area inducing severe errors into the dielectric measurement. As the temperature further increases the means for applying force to the sample eventually displaces all of the sample and rests entirely on the surface of the electrode array thereby terminating the experiment.
Due to the diversity of the two measurement technniques separate instruments have been required for single surface and parallel plate dielectric measurements until now.
A dielectric analyzer is needed which can apply and vary a force to a sample on a single surface electrode sensor and sense the distance between the electrode surface and the means for applying the force.
A dielectric analyzer is also needed which can perform dielectric analysis in both parallel plate and single surface modes within a single instrument.